In recent years, there has been a technique widely researched for crystallizing or enhancing the crystallinity of an amorphous semiconductor film or a crystalline semiconductor film (a semiconductor film having the crystallinity such as poly-crystal or micro-crystal, except a single-crystal), namely, a crystalline semiconductor film which is not single-crystal (hereinafter referred to as a non-single crystalline semiconductor film), formed over an insulating substrate such as glass with laser annealing performed thereto.
For the laser annealing, a pulsed laser oscillator whose output is high such as an excimer laser may be used for example, and it is possible to shape the laser beam into a square spot with several cm on a side or into a linear spot with 10 cm or more in length by an optical system, and besides, the beam spot can irradiate while being moved relative to the irradiation surface to perform the laser annealing. Such a method can enhance the productivity and is industrially superior, and thus it is preferably employed for the actual laser annealing.
In particular, when the linear beam spot is employed, unlike a punctate beam spot requiring to be scanned from front to back and from side to side, the linear beam spot can provide high productivity since an entire surface can be irradiated by scanning the linear beam spot only in a direction perpendicular to the direction of its major axis.
Because of such high productivity, the laser annealing process at present mainly employs the linear beam spot obtained by shaping a beam spot of a pulsed excimer laser with an appropriate optical system.
It is to be noted here that the linear beam spot means a rectangular or elliptical beam spot having a high aspect ratio.
The scan is performed in the direction perpendicular to the direction of the major axis of the linear beam spot because it is the most effective scanning direction.
In order to shape a cross-section of the bean spot into a linear shape on the irradiation surface, an optical system using a cylindrical lens array or the like is generally used in many cases.
The optical system, in addition, not only shapes the cross-section of the beam spot into a linear shape but also homogenizes the energy density distribution of the beam spot on the irradiation surface.
Generally, the optical system for homogenizing the energy density distribution of the laser beam is referred to as a beam homogenizer.
For a beam homogenizer for providing the linear beam spot, a beam progression optical waveguide which is generally referred to as a light pipe may be used (see Patent Document 1), as well as the above-described cylindrical lens array.
This beam progression optical waveguide may be formed in a shape of circular cone, pyramid, column, prism, or the like, and transmits light from one end to the other end by reflection.
[Patent Document 1]
Japanese Patent Laid-Open No. 2004-134785
Two light reflection surfaces are formed in parallel so as to face each other at surfaces along the direction of the beam progression in the beam progression optical waveguide in order to homogenize the energy density distribution of the beam spot in the direction of its minor axis. Thus, a laser beam enters the beam progression optical waveguide from its entrance, and the laser beam is reflected repeatedly on the two light reflection surfaces and is led to its exit.
In other words, the laser beam entered the beam progression optical waveguide is superposed at the exit so as to be folded.
Consequently, the energy density distribution of the laser beams is homogenized at the exit.
The shape of a cross-section of the laser beam emitted from the beam progression optical waveguide depends on the shape of the exit thereof.
Therefore, in the case of obtaining a beam spot whose cross-section has a linear shape, the exit has preferably a linear shape.
FIG. 2A shows a reflective-type beam progression optical waveguide 201, which is a typical example of a conventional beam homogenizer.
In addition, FIG. 2B is a plane schematic diagram showing a light path of a laser beam in the case of using the beam progression optical waveguide 201. A laser beam 202 enters from an entrance 203, the energy density distribution of the laser beam 202 is homogenized by the beam progression optical waveguide, and then the homogenized laser beam is emitted from an exit 204 in FIG. 2B.